Typically, an optical fiber is composed of a central core (i.e., an optical core or optical-fiber core), which transmits and/or amplifies an optical signal, and an inner optical cladding which confines the optical signal within the central core. In this regard, the refractive indices of the central core (nc) and the cladding (ng) are such that nc>ng.
Fiber amplifiers (e.g., fibers doped with rare earth elements) are commonly used for optical applications. For example, erbium doping may be used in optical telecommunication systems for amplifying transmitted optical signals. Such fibers are used in erbium-doped fiber amplifiers (i.e., EDFAs). Erbium-doped fiber amplifiers can have a central core composed of a silica matrix (i.e., a core matrix of silica) that includes doping elements, such as erbium at concentrations of about 250 ppm to 1000 ppm (0.025 to 0.1 weight percent). Furthermore, combining complementary doping elements may improve amplification (e.g., alumina for broadening the gain bandwidth for wavelength dense multiplexing (WDM) applications).
Ytterbium doping is often used in fibers for laser applications. Ytterbium can also be used in erbium-doped fiber amplifiers to improve the effectiveness of absorption of the pump signal by the erbium. Similarly, other rare earth elements can be used alone or in combination depending on the sought applications.
Optical amplification in a rare-earth-doped optical fiber can be achieved by injecting into the optical fiber a pump signal, which excites the rare earth ions (e.g., Er3+ in EDFA). When a light signal passes through this portion of optical fiber, it de-energizes the ions by stimulated emission. In this regard, a photon is produced that is identical in all respects to the incident photon. The light signal is thus doubled. Other rare earth elements (e.g., ytterbium (Yb) and/or thulium (Tm)) can be used as doping elements for amplification of the signal. The other rare earth elements may replace the erbium or be used in combination with the erbium. A portion of such an optical fiber in combination with a resonating cavity formed by a system of mirrors, or Bragg gratings, produces an optical-fiber laser. The wavelength and power of the laser typically depend on the rare earth element used and its concentration.
Amplifying optical fibers are typically produced by incorporating rare earth ions in the silica matrix of the central core (i.e., the core matrix). The rare earth ions incorporated into the central core are typically accompanied by other dopants for improving the amplification gain, widening the amplification band, and/or limiting inhomogeneities in the dispersion of the rare earth dopants in the core matrix. For example, the rare earth dopants may be accompanied by alumina (Al2O3) and/or phosphorus (P). Exemplary amplifying optical fibers may also contain germanium (Ge) in the central core to ensure the refractive index profile (e.g., a step index profile) necessary for guiding and confining the transmitted signal in the central core.
Signals transmitted in optical fibers typically undergo optical losses that accumulate over the distance traveled. These transmission losses increase substantially when the optical fiber is subjected to ionizing radiation such as beta, alpha, gamma rays, and/or X-rays. The optical fiber may be subjected to such radiation when it is used in an optical system containing ionizing radiation (e.g., a nuclear power plant, a particle acceleration laboratory or a satellite in space). In such an environment, the ionizing radiation can reach levels greater than or equal to 100 gray. For example, the radiation in a space environment can reach levels of 10,000 rad, while the radiation in nuclear power plants may be on the order of a megagray (108 rad).
Typically, when conventional radiation-resistant optical fibers are used in radioactive environments, the transmitted optical signal is amplified using electronic systems (i.e., amplifying optical fibers are typically not used). However, a system for amplifying optical signals in radioactive environments that does not use electronic systems (i.e., an all-optical system) would be more desirable.
Passive optical fibers (i.e., fibers that are not doped with rare earth ions) have been designed to be used in radioactive environments. For example, U.S. Pat. No. 4,690,504, which is hereby incorporated by reference in its entirety, discloses an exemplary optical fiber without germanium (Ge) in its core. The absence of germanium in the central core improves the optical fiber's resistance to ionizing radiation. The inner optical cladding is doped (e.g., with fluorine) to reduce the refractive index of the silica. This patent also discloses an exemplary embodiment of an optical fiber having a core, which is lightly doped with fluorine to compensate for a surplus of oxygen in the core.
U.S. Pat. No. 5,509,101, which is hereby incorporated by reference in its entirety, discloses an exemplary optical fiber that is resistant to X-rays and gamma rays. The central core and cladding of this exemplary optical fiber are doped with fluorine. This patent describes several exemplary embodiments with different concentrations of fluorine and germanium. This patent also suggests that including germanium in the central core may reduce transmission losses.
Tammela et al., “Direct Nanoparticle Deposition process for manufacturing very short high gain Er-doped silica glass fibers,” ECOC 2002; 28th European Conference on optical communication; IEEE; Piscataway, N.J., U.S.A., Vol. 4, 2002, page 2, which is hereby incorporated by reference in its entirety, discloses an exemplary vapor-based process in which glass forming elements and dopants are reacted in a torch to manufacture highly erbium-doped fibers to be used in compact amplifiers.
International Publication No. 2005/109055 (and its counterpart U.S. Pat. No. 6,947,650), which is hereby incorporated by reference in its entirety, discloses an exemplary optical fiber with a pure silica core and a cladding doped with fluorine. This document suggests that a high ratio between the diameters of the inner optical cladding and the core (e.g., between about 9 and 10) may improve the resistance of the fiber to ionizing radiation.
Typically, fibers with a pure silica core, or a central core doped with fluorine, exhibit smaller losses in a radioactive environment compared to (i) fibers having a silica core doped with germanium, and (ii) fibers containing phosphorus in the central core or cladding. Amplifying fibers typically require rare earth dopants in the central core, and may include dopants for improving the gain of the optical fiber. However, the incorporation of these dopants in the central core of an amplifying fiber leads to significant losses when the fiber is subjected to ionizing radiation. Additionally, the presence of alumina or phosphorus in an optical fiber leads to an increase in optical losses when the fiber is subjected to ionizing radiation.
The publication of H. Henschel et al., “Radiation-Induced loss of Rare Earth doped silica fibers”, IEEE 1998, pp. 439-444, which is hereby incorporated by reference in its entirety, recognizes the problem of increased losses in amplifying fibers in a radioactive environment. This publication proposes limiting the concentrations of dopants, but fails to specify a method for manufacturing a radiation-resistant amplifying optical fiber.
U.S. Patent Publication No. 2003/175003, which is hereby incorporated by reference in its entirety, describes an exemplary method for producing an amplifying optical fiber in which the rare earth elements are introduced into the central core by the incorporation of nanoparticles. The matrix of the nanoparticles (i.e., the nanoparticle matrix) has a composition different from that of the central core. The nanoparticles have an alumina (Al2O3) or antimony trioxide (Sb2O3) matrix doped with erbium. However, such nanoparticles are not designed to resist radiation.
A need therefore exists for an amplifying or laser fiber, which can be used in an environment with strong ionizing radiation with limited optical losses.